The present invention relates in general to an information recording and playback apparatus; and, more particularly, to a control method of a laser diode for writing to or reading from an optical disc used in a camcorder.
It has been long since a video camera using an optical disc device as a recording medium was produced. The optical disc device itself is now expanding its application from compact disc (CD) and digital versatile disc (DVD) towards a next generation DVD such as high definition DVD (HD DVD) and Blu-ray disc (BD). The next generation DVD has a recording capacity three to five times greater than traditional DVDs, and recent advances in plasma display panel (PDP) and increasing demand for high definition image has aroused a lot of interest in it as a recording medium to cope with an increase in data rate along with the high definition image.
In the optical disc device, an increase in recording capacity per unit area is one of factors of increasing the recording capacity. To accomplish this, it is necessary to make a laser beam irradiated onto an optical disc to record or read data smaller in diameter. The diameter of laser beam can be reduced simply by using a shorter wavelength of laser beam for writing or reading.
Among laser diode (LD) light sources, a blue-violet laser diode is known to output the light with the shortest wavelength. Examples of products that have an optical disc device using the blue-violet laser diode include PC (Personal Computer), game device, video recorder, and so on. Also, there is a laser beam image forming apparatus that uses a laser diode.
A disadvantage of the above-described optical disc device or an apparatus using the same is that if temperature of a laser diode is lower than ambient temperature, dews are sometimes formed on the output side of the laser diode and energy of the laser beam is converted into heat energy by the waterdrops, possibly breaking a lens. To prevent this, JP-A-2000-040850 or JP-A-2000-037906 suggests that if dew condensation takes place or LD temperature is lower than a preset temperature, a laser beam should not be outputted until dew condensation is eliminated by feeding an offset current equal to or lower than a threshold level.
In addition, JP-A-2004-171655 describes that photographing operation of an optical disc device used for a camcorder can be assured by lowering false detection of dew condensation on a laser diode of the optical disc device.
However, short wavelength LD, e.g., blue-violet LD has a narrower operation guarantee temperature range than LDs of different colors with long wavelengths, and does not operate at a low temperature. Also, a low temperature kink phenomenon or long-term rise in laser beam intensity may occur. The low temperature kink phenomenon is observed when current-laser power linearity breaks down, given that current (horizontal axis) fed to a LD and laser beam output (vertical axis) characteristics are plotted with temperature as a parameter. The long-term rise in laser beam intensity means that it takes a long time to increase a laser beam emitted from an LD to which current had been applied up to a specific intensity level.
These problems rarely occur when camcorders or portable BD players are used indoor, but recording (write) or reading information may not be possible if they are used in cold outdoor areas.
FIG. 1 graphically illustrates a relationship between supply current (or simply current) I and output laser power L (I-L characteristics) when LD temperature is 25° C. The horizontal axis represents current values (unit: [mA]) fed to an LD, and the vertical axis represents laser power (unit: [mW]) of a laser beam output corresponding to the current value being supplied.
In FIG. 1, Ith indicates a threshold current value, and Isc indicates a maximum allowable current. No laser beam is outputted in area A where the current I is smaller than the threshold current value Ith. When the threshold current value exceeds the current Ith, a laser beam is outputted and linearity is maintained meaning that the laser power increases proportionally to the supply current. Later when the supply current reaches the maximum allowable current Isc, current is no longer fed to the LD such that laser beam output is not increased any more (area B).
Temperature characteristics of an LD will now be described with reference to FIG. 2. FIG. 2 graphically illustrates a relationship between supply current I and output laser power L with laser temperatures (0° C., 10° C., and 20° C.) as a parameter.
As shown in FIG. 2, laser power with respect to the supply current is decreased as LD temperature is increased from 0° C. to 10° C. and 20° C. In addition, a threshold current value tends to increase as temperature is increased.
Meanwhile, when LD temperature is at 0° C., linearity disappears in mid course. The phenomenon of losing linearity at a low temperature is called a low-temperature kink. Since linearity is not present at a temperature where the low-temperature kink is observed, the temperature is outside the operation guarantee temperature range. Typically, LD would not operate at temperature outside the operation guarantee temperature range, it is impossible to write (record) to or read (playback) from an optical disc device.
There is another problem that although LD temperature may have been increased to the operation guarantee temperature, the temperature rise time until a laser beam is outputted tends to get longer at lower temperatures.
Because of this, if LD temperature is low, it can be an option to execute a writing or reading operation anyway despite that a target laser power level required for the writing or reading operation is not yet reached. In this case, however, write quality may be impaired or read error (“misread”) may occur.
JP-A-2000-040850 and JP-A-2000-037906 as related art technologies concerning a laser diode drive controller provided with a cooler are to prevent an LD from being cooled to extremely low temperatures even if the LD may have become very hot by laser beam output. In particular, JP-A-2000-040850 is about how to eliminate trouble in a laser drive controller caused by dew condensation, and JP-A-2000-037906 discloses a technique for driving a disc within a reference temperature range by feeding current equal to or lower than a threshold value since the temperature control is not possible at a low temperature where a cooler does not operate, thereby changing output of a laser beam.
According to JP-A-2004-171655, dew condensation is observed when an optical disc device built in a camcorder is cooled so that an LD therein itself becomes cooled to a temperature even lower than the ambient temperature, causing moisture in ambient atmosphere stuck to the LD. In other words, when LD temperature is higher than the operation guarantee temperature of the LD, a cooling operation is carried out compulsively. This makes only the LD temperature lower than the ambient temperature such that dew condensation problems are accompanied inevitably. This dew condensation phenomenon of JP-A-2004-171655 is also found in JP-A-2000-040850 concerning the countermeasure of dew condensation and in JP-A-2000-037906 concerning a laser diode drive controller with a cooler.
At any rate, the problems in related art techniques illustrated in JP-A-2000-040850, JP-A-2000-037906, JP-A-2004-171655 is not those that cause the low-temperature kink phenomenon such as LD temperature is lower than the operation guarantee temperature range, or the rise in laser beam intensity becomes longer even if the laser diode temperature falls within the operation guarantee temperature range, which are the problems to be solved by the present invention.